Motor vehicles are increasingly equipped with driver assistance systems, which with the aid of sensor systems detect the environment and from the thus recognized traffic situation derive automatic reactions of the vehicle and/or instruct, especially warn the drivers. Here, a distinction is made between comfort and safety functions.
As a comfort function FSRA (Full Speed Range Adaptive Cruise Control) plays the most important roll in the current development. The vehicle adjusts the true speed to the desired speed predefined by the driver, provided the traffic conditions permit this, otherwise the true speed is automatically adapted to the traffic situation.
In addition to an increase of the comfort, safety functions are increasingly the focus, whereby the reduction of the braking and/or stopping distance in emergency situations plays the most important role. The spectrum of the corresponding driver assistance functions extends from an automatic priming of the brake for reducing the brake latency (pre-fill), via an improved brake assistant (BAS+) up to the autonomous emergency braking.
For driver assistance systems of the above described type radar sensor are mainly used today. Also at poor weather conditions they work reliably and can measure in addition to the distance of objects also directly their radial relative speed via the Doppler effect. As transmission frequencies, here 24 and 77 GHz are used.
The antennas of such radar systems are increasingly implemented in planar technology, since this has advantages with regard to costs and size. For the angle determination in azimuth direction both for transmission and/or reception several horizontally offset individual antennas are used and the phases of the associated received signals are evaluated. For the implementation of long-reach functions such as FSRA and collision warning it is important that the offset of the individual antennas is large enough, so that the lateral location of the objects can be determined sufficiently accurate. With known arrangements of the individual antennas they are at maximum as wide as their distance to each other. It is disadvantageous with this arrangement that ambiguities arise in the azimuth angle determination, which can lead to misinterpretations of situations, which in particular is critical for safety functions.